Within the framework of the present invention, the expression low-altitude flight trajectory is understood to mean a flight trajectory allowing an aircraft to follow as closely as possible the terrain overflown, while avoiding any risk of collision with a part of the terrain. Such a trajectory is therefore situated at a predetermined minimum terrain height, for example 500 feet (around 150 meters).
More particularly, although not exclusively, the present invention applies to a tactical military transport plane which exhibits a low thrust/weight ratio and high inertia, and whose maneuvering times are relatively slow in particular with respect to those of a lighter and swifter plane, such as a fighter for example. Moreover, for such a military transport plane, it is in general desirable for said plane not to be easily detectable.
It is known that the calculation of a low-altitude flight trajectory of the aforesaid type requires very significant computational resources for onboard computers used for this purpose.
Now, the crew of an aircraft, in particular of a military transport plane of the aforesaid type, may need to employ a low-altitude flight trajectory, in a quasi-immediate manner, under certain operational flight situations, such as a descent towards the ground in case of unforeseen threat so as to benefit from a masking by the terrain or upon the appearance of IMC (“Instrument Meteorological Conditions”) conditions that are unforeseen in flight at low altitude in VMC (“Visual Meteorological Conditions”) conditions requiring automatic guidance.
Since the computational resources are of course limited on an aircraft, it is generally impossible to construct a lengthy low-altitude flight trajectory within a very short time scale, of the order of a few seconds, as is required in operational flight situations such as those mentioned above.
This drawback is made more acute in respect of tactical military transport planes such as described previously, whose high inertia makes it necessary to compute the low-altitude flight trajectories, with the aid of onboard performance models (climb performance of the aircraft in particular). Such a mode of computation using performance models considerably lengthens the computation times, in particular relative to those intended for a fighter for example, whose high thrust/weight ratio renders the impact of the climb performance less critical.